In some ways, this is just a consequence of a kind of evolutionary leap forward for many applications. We don’t deploy our WARs into application servers anymore, we try to pack a fat JAR, or maybe even a statically linked runtime, into a container. We rebuild the whole world on CI all the time. I mean, Java 8 has issues just respecting cgroup limits, which many a despondent ops engineer has discovered.

I think there’s still room for the 1M SLoC monolith even in this new world, but there are real benefits on the horizon for upgrading to Java 17 and beyond. We’ve reached a point where it’s just more expensive not to make upgrades a regular part of you development cycle. And, no one is stopping you from just staying on 1.8. I mean, I’m sure there are still decaying enterprises stranded on 1.3.

Sadly, i don't get that choice, nor do i get the choice to make the judgement call to leave the monolith on JDK 8 in the name of stability. But hey, at least i'm paid a bit of money for it, so i have some motivation to work on all of it to the best of my abilities, learn a bit more about the JDK internals, have a word or two to tell others about my experience before i inevitably burn out from the churn.

Personally, i do still think that Java is pretty good when it comes to stability and backwards and forwards compatibility, especially since tooling like Maven is actually pretty good when compared to the alternatives (well, the parent POM functionality is confusing sometimes, but oh well, it has its use cases). That said, JDK 8 to newer releases was indeed a generational shift (probably one for the better) and you see similar things with Spring Boot 1.5 to Spring Boot 2.X, those breaking releases are inevitable.

I only wish that the things blocking people from writing more modular systems would disappear over time, so that huge monoliths that are incredibly hard to work with wouldn't be such an issue. I'm not necessarily advocating for microservices here, since people tend to go straight from one ditch into the opposite one (multiple services per person, nightmarish service mesh, needlessly large % of the code being for shuffling data around, suddenly building a distributed system even within a single domain), but at the very least it would be really nice to have someone look at it and say: "Hey, this PDF generation logic looks really brittle and perhaps should be a service of its own." or maybe: "Oh, hey, we're serving our RESTful API calls and front end resources from the same application, maybe we should have a separate front end app, served through a regular web server?"

Then again, if nothing else, i always have the choice to choose where i'm employed, even though i don't feel like letting my coworkers down at the moment either.

This silently broke a bunch of scripts and callbacks because of class cast exceptions and such. The solution was to rewrite the scripts in such a way to import the Java Long class and use that directly to express large integers.

The most important breaking issues I've found were not necessarily in our code base, but in dependencies. Libraries that include libraries that include libraries, all doing their own thing, a lot of the time involving some pretty scary reflection stuff for some unexplained reason. Blindly upgrading libraries only gets you so far, sometimes a dependency gets abandoned and you need to find a replacement and rewrite your code to call the APIs in the right manner to be backwards compatible enough.

This is where it always seems to end up. Why do people bend over so far backwards doing reflection that isn't specified?

(I always found this really frustrating when making the argument for Scala. Yes, Scala-the-language is more complex than Java-the-language. But it's not more complex than the pile of reflection magic that anyone who tries to write a nontrivial system in Java actually ends up using).

Either because there's no other way to do it, or because the official way is too slow.

All that said - for the 'normal' Springboot applications, we saw almost a 25% performance jump for the same code - updated libraries - running on K8. Java 11 was a substantial performance improvement. The jump for us to Java 17 will happen this winter, and looks to be a non-issue so far.

Here's a list of the things that were available in JDK 8 and no longer are: https://advancedweb.hu/a-categorized-list-of-all-java-and-jv...

When you stumble upon something like that, at best you can import a Maven dependency with a version that has what you need, at worst you need to rewrite the code to use another library or set of libraries.

If you have any low level logic like custom class loaders written against older JDK versions (think before 8), then they'll be forwards compatible until 8 for the most part, but will break afterwards. Coincidentally, reading code that deals with low level logic is also not easy to do, especially if it's not commented well.

If you rely upon reflection, or use advanced language features (like the JasperReports framework for generating PDFs, which also has a build step for building the reports), in some cases things might compile but not work at runtime due to class mismatches.

Many frameworks need new major versions to support newer releases than JDK 8, for example Spring Boot 1.5 needs to be upgraded, so you're also dealing with all the changes that are encapsulated by your dependencies. In another project that i also migrated, needed to rewrite a lot of web initialization code for Spring Boot 2.X.

Not only that, but with those framework changes, certain things can break in the actual environments. For example, if you package your app as a far .jar, then you'll no longer be able to serve JSP files out of it. It makes no sense, but packaging it as a .war which can be executed with "java -jar your-app.war" will work for some reason.

I some other libraries, method names remain the same, but signatures change, or sometimes things just get deprecated and removed - you have to deal with all of that, which is especially unfun in code that isn't commented but that the business depends on to work correctly. Throw in external factors such as insufficient coverage of tests and you're in for an interesting time. I'm not saying that it's a type of environment that should be condoned, but it's the objective reality in many software projects.

Oh and i hope that you're also okay with updating all of your app servers (like Tomcat) or JDK installs on the server as well, especially if you depend on some of the Tomcat libraries to be provided and for your frameworks/libraries that depend on them being present at runtime to accept those new versions effortlessly. It all feels very brittle at times.

This is especially a nightmare if your servers were configured manually - personally i'm introducing Ansible and containers to at least isolate the damage of Ops rot, but it's been an uphill battle since day 1.

Here's an exceedingly stupid one: sometimes there are checks in code to make sure that you're using the right version of JDK (or even the Oracle JDK), which get confused with the larger versions. It's easy to fix when it's your code, but really annoying when it's external tools - personal nitpick.

Addendum: here's something that i expected to break, but didn't. We use myBatis as an ORM. It has XML mapper files that define how to map entities and construct queries against the DB based on that. Also, it uses Java interfaces and dynamically calls the underlying XML code as necessary. So essentially you have an interface, which has a corresponding XML file in which you have quasi-Java code (e.g. checking what parameters are passed in to the query) that's used alongside a number of tags to dynamically build SQL queries. Here's an example: https://mybatis.org/mybatis-3/dynamic-sql.html

Instead of something breaking in myBatis, what broke actually was Hibernate in the newer versions of Spring. Oh, and Flyway for DB migrations simply used to work with a particular Oracle DB version as well.

I'm in the middle of a migration to 11 and my general approach has been to get it compiling on 11 while targeting 8 with (--release 8). That seems to catch a great many things that would otherwise fail. The hardest part thus far was figuring out the appropriate libs to replace Java 8 ee stuff, mostly wrt soap and xml. I'm hoping moving to 11 goes smoothly but we'll see. I'm hoping to have the artifacts to a point where they can run on either 8 or 11 without issues.

Mind you; you usually end up with the services all in different states of node/jdk/etc versioning(at least the companies I worked at).

I don't know PrimeFaces, but I have worked with JSP, which does have a flag to set the compiler version (in case you were ever thinking about putting java lambda's in JSP files, which is possible but not recommended due to mixing code).

The hardest part of upgrading the JDK is going from 8 to 9. After 9 it is much more forgivable. As for Spring, major versions are a pain, as is Hibernate. The other parts of Spring are generally well upgradable if you read the Spring changelogs.